1. Field of the Invention
This invention relates generally to the field of dry ice manufacturing, and more particularly to a method and apparatus for producing pellets of dry ice.
2. Description of Related Art
Dry ice is the solid state of carbon dioxide (CO2). There are a vast array of applications for dry ice, including the processing and preservation of meats and other foods. Dry ice is the preferred means of cooling in such applications, since it imparts no color, odor, or taste, and has no lingering deleterious effect on the food. Dry ice also is desirable for the processing of food because its sublimes directly from the solid state to the gaseous phase, leaving no residue behind after yielding its cooling effect; therefore, no clean-up or removal of residual liquid is required. Furthermore, CO2 is neither toxic, poisonous, reactive with other chemicals, nor flammable.
In its solid state, at standard temperature and pressure, carbon dioxide has a constant and stable temperature of xe2x88x92109.33xc2x0 F. Carbon dioxide is normally transported in its liquid state, and stored in refrigerated vessels at a pressure of about 300 psia, and a corresponding temperature of about 0xc2x0 F.
Once the liquid CO2 reaches the manufacturing facility, dry ice is generally formed into one of the two final forms, blocks of dry ice or smaller pellets. Large blocks of dry ice typically are shipped long distances or stored for extended periods, as pellet size pieces sublimate faster.
The basic process for making dry ice blocks from liquid carbon dioxide has long been known. Sometimes, these blocks of dry ice from a block press are reduced to a smaller size that can more easily be handled and used in many types of applications. Other machines, for example the dry ice pelletizer, produces dry ice pellets. Dry ice pellets are easily packaged by the manufacturer and subdivided by the consumer into convenient portions for use. These dry ice pellets find a vast array of applications, including applications in the processing and preservation of meats and other foods because of the thermal, physical, and chemical properties of dry ice. In certain applications, the dry ice pellets come in intimate contact with the food being processed, such as in a meat packing house and in certain seafood processing plants. The dry ice pellets in these applications are delivered directly onto the food being processed to rapidly cool the food and to keep the food below a specified maximum temperature to prevent spoilage while processing and prior to refrigerated storage. Also, dry ice has long been the favored refrigerant for ice cream vendors and distributors.
Conventional dry ice pellet manufacturing processes incorporate several disadvantages and limitations. Prior art arrangements of the injection system and chamber typically dictate the use of only low CO2 flow rates; thus, limiting pellet production. It would be beneficial to provide a pelletizing system that can handle increased flow rates in order to maximize pellet production.
One limitation of known pelletizers, for example, is the angle at which the liquid CO2 is injected into the extrusion chamber. Conventional injection is generally perpendicular to the length (radial centerline) of the extrusion chamber. Such generally perpendicular injection is representatively shown in FIG. 1 of U.S. Pat. No. 5,528,907. Alternatively, an injection path that is generally parallel to the length (radial centerline) of the extrusion chamber has been used. Such generally parallel injection is representatively shown in FIG. 5 of U.S. Pat. No. 5,548,960. Both perpendicular and parallel injection suffer from limited flow stream interaction with the inner wall(s) of the chamber, improper snow piling and clogging problems.
For example, under the generally perpendicular injection conditions, approximately equal amounts (being one-half the total amount) of the injected CO2 flow toward each end of a the chamber after the flow strikes the inside of the chamber. The CO2 enters through the injection port, travels through the core of the chamber and collides into the inner wall of the other side of the chamber approximately normal to the inner wall. The flow then splits into two, opposite directional streams, each flowing toward an end of the chamber. It is problematic that CO2 snow begins to pile up at the collision site, and the pile then grows in length toward either end of the chamber. As snow begins to pile up between the collision site and the vent port, any escaping gaseous CO2 must first travel through this snow pile before it can be released from within the chamber through the vent port. This injection arrangement impedes maximum snow production because pressure builds up in the chamber prematurely as the volume of the chamber ever shrinks from both sides of the injection point due to piled snow, and because pressure does not have an unencumbered path to exit the chamber, but must pass through forming snow. This type of injection also can prematurely clog the exhaust vent(s) of the extrusion chamber with solid CO2, which clogging limits production. This orientation of injection also inefficiently cools the chamber at start-up, delaying the formation of the ice plug, as the injected CO2 cools the chamber from the point of collision out toward the ends. Therefore, the die end of the chamber, the point at which the plug will form, is cooled last.
Another limitation of known pelletizers is the use of only a single injection port that also hampers attempts at increasing injection flow rates into the chamber. Additionally, the geometry of standard injection nozzles is inefficient. The current use of straight, or nontapered, pipe designs of nozzles frequently leads to blockages of the nozzle, completely stopping production. Not only can the non-tapered design clog, but another adverse effect of such a non-tapered pipe is the resultant random pressure variations inside the extrusion chamber. These variations can lead to frequent operator (manual) adjustment of the metering valve.
Further, there is a lack of automation with present pelletizers. An improvement over the conventional injection system and extrusion chamber would be the provision of automated control over the injection of liquid CO2 into the chamber. Current designs have a manually adjustable metering valve that constantly must be adjusted to compensate for numerous operational variables including clogging of the injection port and changes in liquid pressure. Certain high volume dry ice production facilities have many machines producing tons of ice per day. Each one of these machines has at least one of these metering valves and each one of these valves must be adjusted several times per day. Labor cost to monitor and adjust these metering valves is very high. Replacing the manually operated metering valves with automated control process valves would significantly reduce the labor necessary to operate a pelletizer.
Other disadvantages of the conventional dry ice pellet manufacturing processes lie outside the injection system and extrusion chamber of pelletizers. For example, current pelletizing machines do not incorporate an automated start-up procedure. Yet, if injection is orientated for increased production (as the present invention provides), the production of a dry ice plug without manual, time consuming intervention becomes impractical. On machines with six inch bores and larger, the machine on its own may never build a plug. If fact, starting a machine in this manner is very wasteful and dangerous. An automated start-up system would allow the operator to begin the pelletizer run, and not intervene again.
The filter area of present pelletizers is yet another feature in the production of dry ice upon which improvements can be made. Conventional pelletizers have a ratio of filter screen area to chamber bore area that defeats efficient pellet production. As this ratio drops, so too does the production of dry ice. It would be beneficial to provide a pelletizer having a higher ratio of filter screen area to chamber bore area than do conventional pelletizers.
Further, the current piston assemblies of pelletizers are disadvantageous and need improvement to generate better production efficiencies.
Therefore it can be seen that there is a need in the art for an improved dry ice pelletizing system that overcomes these and other prior art deficiencies. It is a provision to such an improved pelletizer that the present invention is primarily directed.
Briefly described, in a preferred form, the present invention is an improved dry ice pellet manufacturing system including an automated helical injection system, an automated start-up system, a chamber having a greater filter screen ratio than prior art designs and a compressing mechanism. The present invention builds upon known pelletizing systems commonly comprising an extrusion chamber having an injection port through which liquid CO2 is introduced into the chamber. In the chamber, the liquid CO2 turns to portions of both gaseous and solid CO2. A piston compresses the CO2 snow in the chamber, and the gaseous CO2 is vented from the chamber through a venting port. The resulting mass of dry ice is then pushed through an extrusion die to produce dry ice pellets.
The automated helical injection system of the present invention comprises compound angle injection, tapered injection nozzles and an automated injection subsystem. Whereas conventional injection nozzles are situated generally perpendicular or parallel to the radial centerline of the extrusion chamber, the present system utilizes compound angle injection into the chamber. Compound angle injection provides the injected CO2 stream with at least an approximately helical flow path within the chamber, the path winding its way to the die end of the chamber. In this way, CO2 snow begins to pile, and is packed, at the die end of the chamber. Thus, there is little or no snow piled between the injection site and the vent port, so pressure can be immediately released. The shortest path between two points on a cylinder (one not directly above the other) is a fractional turn of a helix. An exemplary use of compound angle injection utilizes compound angle nozzles.
Another improvement provided by the present pelletizing system is the use of tapered injection nozzles, wherein the bore of each nozzle diverges in the direction from the metering valve to the extrusion chamber. A diverging injection nozzle as described accelerates the snow through the nozzle, enabling it to pack tighter, squeeze out vapor, and limit clogging.
The present pelletizing system further incorporates a beneficial automated injection subsystem. The automated injection subsystem includes at least two injection ports for injection of liquid CO2 into the chamber, staggered injection rate capability and a valve arrangement.
In order to provide maximum ice production, the present system utilizes both staggered injection rates and a valve arrangement that improve (increases) upon the amount of CO2 injected into the extrusion chamber over time. The automated injection subsystem is similar to the flow of gasoline into a car""s gas tank. At the fill station, an individual places the gas nozzle into the gas pipe, and enables the maximum flow of gas into the tank by pulling the hand lever as hard as possible. When the nozzle senses a preset pressure, the lever is disengaged, and the individual can top off the tank, but only at a reduced flow rate.
The present system utilizes at least two injection flows, a first injection flow that is a maximized flow until a preset pressure within the chamber is reached wherein that injection flow is closed, and a second injection flow of diminished flow rate capable of xe2x80x9ctopping offxe2x80x9d the chamber after the first injection flow is halted.
The valve arrangement provides valves that are adjustable to various flow rates. As the pressure in the chamber increases, the valves are closed in order from highest flow rate to lowest. This arrangement enables the pressure inside of the extrusion chamber to remain at approximately the highest possible pressure below the triple point for most of the injection cycle. These controlled process valves enable the automated injection of liquid CO2. The controlled process valves eliminate the conventional manual labor necessary to adjust the manually operated metering valves of known machines by automating this procedure.
The automated start-up system of the present invention comprises a start-up injection valve that is used to fill the chamber with pressure without blowing snow out of the chamber. The automated start-up system enables the development of an ice plug in the chamber. The compound angle of the start-up injection flow enables the die end of the chamber to cool as fast as possible, as the flow stream is not split, and guided to the die end.
The present invention further utilizes a chamber having a greater filter screen ratio than currently is used in the art. The extrusion chamber of the improved pelletizer of the present invention has approximately a 35% or greater filter screen ratio; filter screen ratio being defined as the ratio of filter screen area to chamber bore area. The chamber can include filter media placed over one or more of the venting ports in order to maximize the vapor exhaust rate of CO2 from the chamber. Filters over the venting ports allow such a rapid exhaust rate without traditional concerns including the loss of snow into the exhaust piping.
The compressing mechanism of the present invention comprises a rod and piston assembly capable of travel within the chamber. The rod can be made of steel, and the piston, a sleeve retainer and sleeve can be made of UHMW polyethylene, TEFLON, DELRIN, oil filled NYLON, NYLON, or any other tough, low-friction, non-stick, non-abrasive, food-grade material.
It will be understood that the above-described benefits of the present invention apply to any dry ice forming apparatus, including block presses and the like, and are not limited only to pelletizers.
Accordingly, it is an object of the present invention to provide an improved method of forming dry ice.
It is another object of the present invention to provide an improved pelletizing system having the above improvements.